This invention relates generally to a control system for a fixed displacement, constant flow pump and more particularly to a hydraulicly actuated electronically controlled unit injector (HEUI) fuel control system using the fixed displacement constant flow pump.
This invention is particularly applicable to and will be described with specific reference to a throttling valve controlling metering of low pressure fluid into a high pressure pump used in a HEUI flow control system. However, the invention has broader application and may be applied to other systems using a constant flow, fixed displacement pump requiring fast response over a wide range of operating conditions such as that which is required in vehicular steering systems.
a) Conventional Systems
As is well known, a hydraulically-actuated electronically-controlled unit injector fuel system has a plurality of injectors, each of which, when actuated, meters a quantity of fuel into a combustion chamber in the cylinder head of the engine. Actuation of each injector is accomplished through valving of high pressure hydraulic fluid within the injector under the control of the vehicle""s microprocessor based electronic control module (ECM).
Generally, sensors on the vehicle impart engine information to the ECM which develops actuator signals controlling a solenoid on the injector and the flow of hydraulic fluid to the injector. The solenoid actuates pressure balanced poppet valves such as shown in U.S. Pat. Nos. 5,191,867 and 5,515,829 (incorporated by reference herein). The poppet valves in the injector port high pressure fluid to an intensifier piston which causes injection of the fuel at very high pressures. The pressure at which the injector injects the fuel is a function of the hydraulic fluid flow supplied the injector by a high pressure pump while the timing of the injector is controlled by the solenoid. Both functions are controlled by the ECM to cause precise pulse metering of the fuel at desired air/fuel ratios to meet emission standards and achieve desired engine performance. Tightening emission standards and a demand for better engine performance have resulted in continued refinement of the control techniques for the injector. Generally the pump flow output has to be variable throughout the operating range of the engine. For example, one manufacturer may desire a constant pump flow throughout an operating engine speed range except at the higher operating engine speeds whereat the injectors are valving so quickly reduced pump flow may be desired even though more fuel is being injected by the injectors to the combustion chambers. Other manufacturers may desire to rapidly change pump flow at any given instant for emission control purposes. For example, the ECM may sense a step load change on the engine and impose a change in the fuel/air ratio to overcome the effects of a transient emission. Still further, the operating vehicular environment severely impacts oil viscosity affecting pump flow and injector performance. Viscosity of the hydraulic fluid is affected by several variables besides heat and is difficult to program into the ECM to fully account for its affect on system performance.
In a HEUI system, high pressure hydraulic actuating fluid is supplied to each injector by a high pressure pump in fluid communication with each injector through a manifold/rail fluid passage arrangement. The high pressure pump is charged by a low pressure pump. As noted in the ""867 patent, the high pressure pump is either a fixed displacement, axial piston pump or alternatively a variable displacement, axial piston pump. If a fixed displacement pump is used, a rail pressure control valve is required to variably control the pressure in the manifold rail by bleeding a portion of the flow from the high pressure pump to a return line connected to the engine""s sump. For example, the ""867 patent mentions varying the output of the high pressure pump by the rail pressure control valve to pressures between 300 to 3,000 psi. A variable displacement pump can eliminate the rail control valve if the flow output of the variable pump can timely meet the response demands imposed by the HEUI system. The pumps under discussion are axial piston pumps in which the pump stroke (displacement) is determined by the angle of the swash plate. Variable displacement, axial piston pumps use various arrangements to change the swash plate angle and thus the piston stroke. Generally speaking, variable output, axial piston pumps do not have the reliability of a fixed displacement, axial piston pump and are more expensive. More significantly, the response time demands for pump output flow in a HEUI system is becoming increasingly quicker and a variable pump may be unable to change output flow within the time constraints of a HEUI system unless a rail pressure control valve is used.
A fixed displacement, high pressure pump is typically used in HEUI systems because of cost considerations. The pump is sized to match the system it is applied to. It is well known that the flow of a fixed displacement pump increases, generally linearly, with speed. Accordingly, the fixed displacement pump is sized to meet HEUI system demands at a minimal engine speed which is less than the normal operating speed ranges of the engine. Higher engine speeds produce excess pump flow which is dumped by the rail pressure control valve to return. The excess flow represents an unnecessary power or parasitic drain on the engine which the engine manufacturers have continuously tried to reduce.
For example, U.S. Pat. No. 5,957,111 shows a control scheme in which excess pump flow is passed to an idle injector but at a rate insufficient to actuate the injector. The system is stated to allow elimination of the rail pressure control valve and permit a more accurate sizing of the fixed displacement pump. However, the system does not avoid unnecessary parasitic engine power drains imposed by the pump. The pump must still be sized to produce a set flow sufficient to actuate the injectors at a low speed and that flow increases with pump speed.
b) The Parent Patent
The parent patent (incorporated herein by reference in its entirety herein) discloses a fixed displacement, axial pump which in contrast to conventional axial piston pumps, eliminates the kidney shaped ports, rotates the cylinder, fixes the swash plate against rotation and establishes an orificed, suction slot inlet for each piston. The suction slot draws a constant volume of fluid into each pump cylinder once pump operating speed is reached to produce a constant flow output from the pump. The pump can therefore be designed to produce the maximum flow required by the HEUI system (i.e., at low operating speeds) which maximum does not increase when pump speed increases as in fixed displacement, axial piston pumps. The power otherwise expended to drive conventional fixed displacement pumps beyond their designed xe2x80x9cmaximumxe2x80x9d is not required. Improved vehicle performance, better fuel consumption and decreased emissions results because the parasitic power drain is removed.
Additionally, and as noted above, there are times during the vehicle""s operation where less flow from the required xe2x80x9cmaximumxe2x80x9d is sufficient to operate the injectors and desired for better injector performance, enhanced fuel consumption, etc. In the parent application, it was demonstrated that controlling the flow of fluid to the constant volume high pressure pump by a throttling valve could produce a constant pump output flow at any desired level. The results and benefits achieved by the constant flow pump as discussed above relative to the maximum output sizing consideration, can therefore be achieved throughout the operating range of the pump by a throttling valve at the pump inlet. Parasitic power drains on the system are thus alleviated over the entire operating range of the engine.
The throttling valve generally disclosed in the parent application was simply a solenoid operated valve under the control of the ECM and similar to the high pressure, axial pressure control valve (RPCV) currently used in conventional systems. Because the solenoid valve is controlling the flow of a low pressure pump, its sizing is reduced decreasing its cost. While the solenoid operated valve can throttle the flow to the inlet of the constant flow pump, the viscosity changes in the hydraulic fluid such as the variations that can occur between ambient vehicular start-up temperatures and the sudden fluid flow changes occurring during normal operating conditions, such as that occurring during vehicle acceleration or deceleration, impose requirements on a conventional solenoid valve which are difficult to achieve.
Accordingly, one of the major undertakings of this invention is to provide a throttling valve for the constant flow pump inlet which is responsive to the various demands imposed on the pump by the system, particularly a HEUI system.
This feature along with other advantages of the invention is achieved in an internal combustion engine having a hydraulically-actuated electronically-controlled fuel injection system of the type including a fuel injector valving high pressure fluid in response to commands from an electronic command module (ECM) to timely inject a metered quantity of fuel into the engine""s combustion chamber. The injector is in fluid communication with the outlet of a high pressure pump in turn having an inlet in fluid communication with a low pressure pump. This system includes, in the preferred embodiment, an axial piston, fixed displacement high pressure pump producing a generally constant flow of fluid throughout the operating range of the high pressure pump, but in a broader sense, covers any pump which can be throttled controlled at the pump inlet. Coupled to such high pressure pump is a throttling valve having an inlet in fluid communication with the low pressure pump and an outlet in fluid communication with the high pressure pump. The throttling valve has a flow control slave valve providing a variably set flow from the throttling valve inlet to the throttling valve outlet and a pilot operated spool valve controlling the variably set flow of the slave valve whereby the flow rate of the pump may be variably set to a desired flow within a large operating condition range.
In accordance with another aspect of the invention, the spool valve includes a regulating valve for exerting a generally constant pressure on the spool valve tending to further increase the set flow of the slave valve and a solenoid actuated pressure control flow exerting a pressure on the spool valve acting opposite to the constant pressure whereby the spool valve functions as a hydraulically unbalanced, mechanical actuator controlling the slave valve to achieve a throttling valve more responsive to command positions than that which can be achieved by a direct actuated valve such as a solenoid actuated poppet valve. More particularly, the solenoid valve is somewhat isolated from viscosity variations in the pump oil because its function is to simply create a pressure for the actuator. It is not exposed to oil flow forces through the valve which vary with viscosity changes.
In accordance with another feature of the invention, the slave valve includes a slave valve housing containing a flow valve passage therein and a longitudinally extending, cylindrical sleeve within the flow valve passage having an inlet opening in fluid communication with the inlet and an outlet opening in fluid communication with the outlet. A cylindrical hollow piston having a closed end is positioned within the sleeve. The sleeve or the piston has a plurality of longitudinally spaced orifice openings of set variable size extending therethrough for providing fluid communication from the throttling valve inlet to the throttling valve outlet through select orifice openings in registry with either the inlet or outlet openings as a result of piston position. A spring biases the piston to a stop position acting against the bias of the unbalanced mechanical actuator. By sizing select orifice openings (dimensional size and longitudinal distance), the area of the slave valve can remain in a full open position for a set travel of the piston to compensate for viscosity variations in the pump oil during engine warm-up.
It is thus one object of the invention to provide a pilot operated throttling valve for an inlet of any constant flow axial piston pump used in any system which is capable of controlling flow to the pump over a wide range of flow rates and fluid viscosities.
Another feature of the invention is to provide a throttling valve of the type generally described in a HEUI system which uses a solenoid valve controlled by the ECM that can be sized smaller and consequently be less expensive than that required of a solenoid valve functioning as a direct throttling valve.
Still another important object of the invention is to provide an improved HEUI system that uses a constant flow, axial piston pump with a pilot operated throttling inlet valve that accomplishes one or more or any combination of the following:
a) elimination or reduction of parasitic power drains on the engine thereby producing improved power or performance, better fuel economy, less emissions, etc.;
b) generally full flow at start-up and during warm-up independent of viscosity and flow rate variation;
c) minimal flow, or optionally, full flow upon electrical system failure;
d) minimize adverse effects on destroking the pump; and,
e) generally excellent response to ECM commands permitting stable and controllable HEUI operation and/or future developments or enhancements of HEUI systems.
These and other objects, features and advantages of the invention will become apparent to those skilled in the art upon reading and understanding the Detailed Description of the Invention set forth below.